# Melodia in Python: A Tutorial

This tutorial shows how to use the Melodia Melody Extraction vamp plugin directly in python.

• The tutorial assumes working knowledge of python (and that python is already installed on your system).

J. Salamon and E. Gómez, "Melody Extraction from Polyphonic Music Signals using Pitch Contour Characteristics", IEEE Transactions on Audio, Speech and Language Processing, 20(6):1759-1770, Aug. 2012. (copyright notice)

Created by Justin Salamon, January 2016.

## Step 1: Install Melodia

Tip: and easy way to check if the plugin is installed correctly is to open Sonic Visualiser and then check if the plugin appears in the menu under Transform > Analysis by Category > Pitch > MELODIA - Melody Extraction...

## Step 2: Install the vamp python module

>pip install vamp

Tip: if you're not already using pip to manage your python packages: https://pip.pypa.io/en/stable/

## Step 3: Using Melodia in python

### 3.1 Necessary modules

We need to load the vamp module of course. Since the module doesn't handle audio loading, we also need something to do that. There are many ways to load audio into python, I normally use either Essentia or Librosa. If you don't have neither installed, Librosa is probably easier to set up (pip install librosa). Finally, we'll use matplotlib for visualizing the extracted melody pitch contour.

In [1]:
import vamp
import librosa
import essentia.standard as es
import matplotlib.pyplot as plt
%matplotlib inline
from __future__ import print_function


In [2]:
# This is the audio file we'll be analyzing.
audio_file = '/Users/justin/datasets/melody/mirex05/audio/train05.wav'

In [3]:
# This is how we load audio using Essentia

In [4]:
# This is how we load audio using Librosa
audio, sr = librosa.load(audio_file, sr=44100, mono=True)


### 3.3 Exracting the melody using Melodia with default parameter values

In [5]:
data = vamp.collect(audio, sr, "mtg-melodia:melodia")

In [6]:
# data is a dictionary containing one item called "vector"
data

Out[6]:
{'vector': ( 0.002902494,
array([-220., -220., -220., ..., -220., -220., -220.], dtype=float32))}
In [7]:
# vector is a tuple of two values: the hop size used for analysis and the array of pitch values
# Note that the hop size is *always* equal to 128/44100.0 = 2.9 ms
hop, melody = data['vector']
print(hop)
print(melody)

 0.002902494
[-220. -220. -220. ..., -220. -220. -220.]


### \*\*\* SUPER IMPORTANT SUPER IMPORTANT \*\*\*

For reasons internal to the vamp architecture, THE TIMESTAMP OF THE FIRST VALUE IN THE MELODY ARRAY IS ALWAYS:

first_timestamp = 8 * hop = 8 * 128/44100.0 = 0.023219954648526078

This means that the timestamp of the pitch value at index i (starting with i=0) is given by:

timestamp[i] = 8 * 128/44100.0 + i * (128/44100.0)

So, if you want to generate a timestamp array to match the pitch values, you do it like this:

In [8]:
import numpy as np
timestamps = 8 * 128/44100.0 + np.arange(len(melody)) * (128/44100.0)


As noted above, the hop size used in Melodia is always 2.9 ms, regardless of the sampling rate of the audio being analysed (though a rate of 44100 is recommended for optimal performance). This in turn means the first timestamp is always 23.2 ms, again, regardless of the sampling rate.

### 3.4 Extracting the melody using Melodia with custom parameter values

In [9]:
# parameter values are specified by providing a dicionary to the optional "parameters" parameter:
params = {"minfqr": 100.0, "maxfqr": 800.0, "voicing": 0.2, "minpeaksalience": 0.0}

data = vamp.collect(audio, sr, "mtg-melodia:melodia", parameters=params)
hop, melody = data['vector']


Melodia has 4 parameters:

• minfqr: minimum frequency in Hertz (default 55.0)
• maxfqr: maximum frequency in Hertz (default 1760.0)
• voicing: voicing tolerance. Greater values will result in more pitch contours included in the final melody. Smaller values will result in less pitch contours included in the final melody (default 0.2).
• minpeaksalience: (in Sonic Visualiser "Monophonic Noise Filter") is a hack to avoid silence turning into junk contours when analyzing monophonic recordings (e.g. solo voice with no accompaniment). Generally you want to leave this untouched (default 0.0).

### 3.4 Plot the extracted melody

In [10]:
# Melodia returns unvoiced (=no melody) sections as negative values. So by default, we get:
plt.figure(figsize=(18,6))
plt.plot(timestamps, melody)
plt.xlabel('Time (s)')
plt.ylabel('Frequency (Hz)')
plt.show()

In [11]:
# A clearer option is to get rid of the negative values before plotting
melody_pos = melody[:]
melody_pos[melody<=0] = None
plt.figure(figsize=(18,6))
plt.plot(timestamps, melody_pos)
plt.xlabel('Time (s)')
plt.ylabel('Frequency (Hz)')
plt.show()

In [12]:
# Finally, you might want to plot the pitch sequence in cents rather than in Hz.
# This especially makes sense if you are comparing two or more pitch sequences
# to each other (e.g. comparing an estimate against a reference).
melody_cents = 1200*np.log2(melody/55.0)
melody_cents[melody<=0] = None
plt.figure(figsize=(18,6))
plt.plot(timestamps, melody_cents)
plt.xlabel('Time (s)')
plt.ylabel('Frequency (cents relative to 55 Hz)')
plt.show()


## That's all folks

For further information feel free to contact me: [email protected] (or [email protected])